The economical manufacture of chemicals using photochemical reactions induced by laser radiation as one of the production steps is a very active field of research. Although laser energy is currently expensive by comparison with thermal energy, the high degree of control over the reaction products possible with photochemical reactions means that for high value products such as isotopes or pharmaceuticals use of such reactions may be an economical method of production. A fundamental concern in the design of a plant using laser induced photochemical reactions is the design of the reaction cell where the photochemical reactions take place.
Conventional reaction cells are of three main types, those which use free space focussed radiation, those with conducting walls, and equifluence cells. The simplest design for a reaction cell for photochemical reactions which require a fluence in excess of the damage threshold of available window materials is one utilizing free space focussed radiation. In this design, the laser beam enters the cell as a converging beam of large diameter, reaches a focus within the cell and then diverges again before leaving the cell through the exit window. The drawbacks of these cells are:
(a) Only a small volume of the cell around the focus is exposed to high fluence before the beam diverges again, leading to a small yield/pulse.
(b) The long path length through the cell before reaching the high fluence region where photochemical reactions take place compared to the short path length within the high fluence region means that the yield/photon is also small.
(c) Because of the small yield per pulse, multipulse irradiations are required to induce photochemical reactions in a large fraction of the material in the cell. During multipulse irradiations, secondary reactions (photochemical or otherwise) can occur which interfere with the desirable primary reactions taking place.
The use of a waveguide with conducting walls has been reported previously. The drawbacks of such cells are the expense involved in coating the cell wall with a suitable material such as gold, the high probability of reactions between metal surfaces and the process gas or the photochemical reaction products, and the ease with which such surfaces can be degraded by dirt, requiring time consuming cleaning or recoating procedures.
Another problem is the extreme difficulty of using a metal waveguide for the purpose without suffering high losses. A circular section waveguide will propagate radiation in three modes: circular electric, circular magnetic, and hybrid in the case of a metal waveguide the circular magnetic and the hybrid modes are high loss hodes, and so only the circular electric mode is usable. However, the circular electric mode is extremely difficult to couple to free space radiation and its application to the process described herein would be uneconomic because of the high coupling loss that would be encountered in practice.
For any photochemical reaction there is generally an optimal fluence (laser energy per unit area). Reaction cells have been proposed which achieve a close approximation to this optimal fluence throughout their volume; such cells are called equifluence cells. Although equifluence cells can achieve close to the theoretical maximum efficiency, their use has a number of drawbacks:
(a) The mirrors used in equifluence cell design have complex aspheric geometries. Such mirrors are costly to manufacture.
(b) The design of equifluence cells generally requires that the aspheric mirrors are in contact with the process gas. Reactions between the process gas or photochemical reaction products and the mirror material can then lead to a reduced lifetime for these mirrors.
(c) If the cell is to be truly equifluence, the mirrors must be exposed to the same fluence as the process gas. If this fluence is high (as would be required for instance in a process to remove CF.sub.3 T from CF.sub.3 H by multiphoton dissociation using a CO.sub.2 laser), there may be no materials available which can be exposed to this fluence without damage.
The present invention provides a relatively simple, inexpensive waveguide reaction cell which largely overcomes the drawbacks discussed above and offers significant advantages over existing designs. The invention also provides a process for inducing photochemical reactions in gases using laser radiation, which is made possible by the use of such a cell.